But MIT’s Robust Robotics Group — which fielded the team that won the last AUVSI contest — has set itself an even tougher challenge: developing autonomous-control algorithms for the indoor flight of GPS-denied airplanes. At the 2011 International Conference on Robotics and Automation (ICRA), a team of researchers from the group described an algorithm for calculating a plane’s trajectory; in 2012, at the same conference, they presented an algorithm for determining its “state” — its location, physical orientation, velocity and acceleration. Now, the MIT researchers have completed a series of flight tests in which an autonomous robotic plane running their state-estimation algorithm successfully threaded its way among pillars in the parking garage under MIT’s Stata Center.

“The reason that we switched from the helicopter to the fixed-wing vehicle is that the fixed-wing vehicle is a more complicated and interesting problem, but also that it has a much longer flight time,” says Nick Roy, an associate professor of aeronautics and astronautics and head of the Robust Robotics Group. “The helicopter is working very hard just to keep itself in the air, and we wanted to be able to fly longer distances for longer periods of time.”

With the plane, the problem is more complicated because “it’s going much faster, and it can’t do arbitrary motions,” Roy says. “They can’t go sideways, they can’t hover, they have a stall speed.”

Found in Translation

To buy a little extra time for their algorithms to execute, and to ensure maneuverability in close quarters, the MIT researchers built their own plane from scratch. Adam Bry, a graduate student in the Department of Aeronautics and Astronautics (AeroAstro) and lead author on both ICRA papers, consulted with AeroAstro professor Mark Drela about the plane’s design. “He’s a guy who can design you a complete airplane in 10 minutes,” Bry says. “He probably doesn’t remember that he did it.” The plane that resulted has unusually short and broad wings, which allow it to fly at relatively low speeds and make tight turns but still afford it the cargo capacity to carry the electronics that run the researchers’ algorithms.

Because the problem of autonomous plane navigation in confined spaces is so difficult, and because it’s such a new area of research, the MIT team is initially giving its plane a leg up by providing it with an accurate digital map of its environment. That’s something that the helicopters in the AUVSI challenges don’t have: They have to build a map as they go.

But the plane still has to determine where it is on the map in real time, using data from a laser rangefinder and inertial sensors — accelerometers and gyroscopes — that it carries on board. It also has to deduce its orientation — how much it’s tilted in any direction — its velocity, and its acceleration. Because many of those properties are multidimensional, to determine its state at any moment, the plane has to calculate 15 different values.

That’s a massive computational challenge, but Bry, Roy and Abraham Bachrach — a grad student in electrical engineering and computer science who’s also in Roy’s group — solved it by combining two different types of state-estimation algorithms. One, called a particle filter, is very accurate but time consuming; the other, called a Kalman filter, is accurate only under certain limiting assumptions, but it’s very efficient. Algorithmically, the trick was to use the particle filter for only those variables that required it and then translate the results back into the language of the Kalman filter.

Confronting Doubt

To plot the plane’s trajectory, Bry and Roy adapted extremely efficient motion-planning algorithms (http://web.mit.edu/newsoffice/2011/smarter-robot-arms-0921.html) developed by AeroAstro professor Emilio Frazzoli’s Aerospace Robotics and Embedded Systems (ARES) Laboratory. The ARES algorithms, however, are designed to work with more reliable state information than a plane in flight can provide, so Bry and Roy had to add an extra variable to describe the probability that a state estimation was reliable, which made the geometry of the problem more complicated.

Paul Newman, a professor of information engineering at the University of Oxford and leader of Oxford’s Mobile Robotics Group, says that because autonomous plane navigation in confined spaces is such a new research area, the MIT team’s work is as valuable for the questions it raises as the answers it provides. “Looking beyond the obvious excellence in systems,” Newman says, the work “raises interesting questions which cannot be easily bypassed.”

But the answers are interesting, too, Newman says. “Navigation of lightweight, dynamic vehicles against rough prior 3-D structural maps is hard, important, timely and, I believe, will find exploitation in many, many fields,” he says. “Not many groups can pull it all together on a single platform.”

The MIT researchers’ next step will be to develop algorithms that can build a map of the plane’s environment on the fly. Roy says that the addition of visual information to the rangefinder’s measurements and the inertial data could make the problem more tractable. “There are definitely significant challenges to be solved,” Bry says. “But I think that it’s certainly possible.”

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Now researchers at MIT, Harvard University and Seoul National University have engineered a soft autonomous robot that moves via peristalsis, crawling across surfaces by contracting segments of its body, much like an earthworm. The robot, made almost entirely of soft materials, is remarkably resilient: Even when stepped upon or bludgeoned with a hammer, the robot is able to inch away, unscathed.

Sangbae Kim, the Esther and Harold E. Edgerton Assistant Professor of Mechanical Engineering at MIT, says such a soft robot may be useful for navigating rough terrain or squeezing through tight spaces.

The robot is named “Meshworm” for the flexible, meshlike tube that makes up its body. Researchers created “artificial muscle” from wire made of nickel and titanium — a shape-memory alloy that stretches and contracts with heat. They wound the wire around the tube, creating segments along its length, much like the segments of an earthworm. They then applied a small current to the segments of wire, squeezing the mesh tube and propelling the robot forward. The team recently published details of the design in the journal IEEE/ASME Transactions on Mechatronics.

In addition to Kim, the paper’s authors are graduate student Sangok Seok and postdoc Cagdas Denizel Onal at MIT, associate professor Robert J. Wood at Harvard, assistant professor Kyu-Jin Cho PhD ’07 of Seoul National University, and Daniela Rus, professor of computer science and engineering and director of MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL).

Soft-Serve Robotics

In the past few decades, many roboticists have looked for ways to engineer soft robotic systems. Without bulky, breakable hardware, soft robots might be able to explore hard-to-reach spaces and traverse bumpy terrain. Their pliable exteriors also make them safe for human interaction.

A significant challenge in soft robotics has been in designing soft actuators, or motors, to power such robots. One solution has been to use compressed air, carefully pumped through a robot to move it. But Kim says air-powered, or pneumatic, robots require bulky pumps. “Integrating micro air compressors into a small autonomous robot is a challenge,” Kim says.

Artificial Muscle from a Bizarre Material – (Nickel Titanium)

Instead, Kim and his colleagues looked to the earthworm for design guidance. They noted that the creepy crawler is made up of two main muscle groups: circular muscle fibers that wrap around the worm’s tubelike body, and longitudinal muscle fibers that run along its length. Both muscle groups work together to inch the worm along.

The team set out to design a similar soft, peristalsis-driven system. The researchers first made a long, tubular body by rolling up and heat-sealing a sheet of polymer mesh. The mesh, made from interlacing polymer fibers, allows the tube to stretch and contract, similar to a spring.

They then looked for ways to create artificial muscle, ultimately settling on a nickel-titanium alloy. “It’s a very bizarre material,” Kim says. “Depending on the [nickel-titanium] ratio, its behavior changes dramatically.”

Depending on the ratio of nickel to titanium, the alloy changes phase with heat. Above a certain temperature, the alloy remains in a phase called austenite — a regularly aligned structure that springs back to its original shape, even after significant bending, much like flexible eyeglass frames. Below a certain temperature, the alloy shifts to a martensite phase — a more pliable structure that, like a paperclip, stays in the shape in which it’s bent.

The researchers fabricated a tightly coiled nickel-titanium wire and wound it around the mesh tube, mimicking the circular muscle fibers of the earthworm. They then fitted a small battery and circuit board within the tube, generating a current to heat the wire at certain segments along the body: As a segment reaches a certain temperature, the wire contracts around the body, squeezing the tube and propelling the robot forward. Kim and his colleagues developed algorithms to carefully control the wire’s heating and cooling, directing the worm to move in various patterns.

The group also outfitted the robot with wires running along its length, similar to an earthworm’s longitudinal muscle fibers. When heated, an individual wire will contract, pulling the worm left or right.

As an ultimate test of soft robotics, the group subjected the robot to multiple blows with a hammer, even stepping on the robot to check its durability. Despite the violent impacts, the robot survived, crawling away intact.

“You can throw it, and it won’t collapse,” Kim says. “Most mechanical parts are rigid and fragile at small scale, but the parts in Meshworms are all fibrous and flexible. The muscles are soft, and the body is soft … we’re starting to show some body-morphing capability.”

Kellar Autumn, a professor of biology at Lewis and Clark College, studies the biomechanics of animal motion in designing soft robotics. Autumn says robots like the Meshworm may have many useful applications, such as next-generation endoscopes, implants and prosthetics.

“Even though the robot’s body is much simpler than a real worm — it has only a few segments — it appears to have quite impressive performance,” Autumn says. “I predict that in the next decade we will see shape-changing artificial muscles in many products, such as mobile phones, portable computers and automobiles.”

This research was supported by the U.S. Defense Advanced Research Projects Agency.

Links

http://web.mit.edu/newsoffice/2012/autonomous-earthworm-robot-0810.html
Youtube Video – http://youtu.be/EXkf62qGFII

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