Rockin' and Rollin': Tensegrity Locomotion

Rolling is the new walking, judging from publications in tensegrity robotics.

Super Ball Bot by NASA robotics scientists Vytas SunSpiral and Adrian Agogino works in this way. The rolling is enabled by changing the cable lengths, as can be seen in this CAD video:


John Wenz covered the team in Popular Mechanics, and wrote:

The Super Ball Bot by NASA robotics scientists Vytas SunSpiral and Adrian Agogino is capable of going where rovers and stationary probes cannot. Obstacles such as rocks or soft soil, the kind of problems that plague the Mars Spirit rover can't stop the structure. It doesn't have wheels in the traditional sense. Rather, the probe is one giant wheel capable of bounding along the surface, with the added capability of using its rods to push and pull itself out of complex terrain—and even walk as needed. This is all accomplished by controlling the cables to change shape and create motion. 

Super Ball Bot works on a concept known as a tensegrity structure, and SunSpiral and Agogino's design draws a little inspiration from futurist Buckminster Fuller and artist Kenneth Snelson. SunSpiral compares the mechanics of tensegrity to a skeletal structure. Rather than having rigid connections, such as a wheel on an axle, the ball's network of rods and cables is under tension but has no rigid hinges, and the components are free to move relative to each other. "They're under compression, they're being squeezed, they're doing their job as rods, but they don't touch each other, so the way that force propagates through the tensegrity structure is very different," SunSpiral says. "Forces diffuse through it, and they have some very interesting properties, because tension elements behave very differently. You don't have lever arms in them, so you get a lot of neat qualities such as this very inherent compliance and flexibility in the structure that is exactly what you need in a robot that's really going to interact with the real world." 

While Sunspiral has NASA's attention, other teams are working more quietly on prototypes. Mechanical and Computer Engineers at Union College, Schenectady published a paper documenting their success with a vibration powered tensegrity robot. They wrote,

Tensegrities are an appealing platform for modern robotics. They are robust, agile, and can quickly change shape, lending themselves to promising applications ranging from urban search-and-rescue to biomedical devices. However, these properties also make them exceedingly difficult to control through conventional means, particularly as the complexity of the robot increases. We have described a means of actuating and controlling tensegrity robots which treats their dynamical complexity as a feature to be exploited rather than as a liability to be suppressed. By designing the structure in order to maximize resonant possibilities, we can make the robot move simply by vibrating it at specific frequencies. This leads to a tensegrity robot which is much smaller and much simpler than existing designs, and yet outperforms in many regards. 

More valuably, we have demonstrated how we can affect behavioral change merely by changing the frequencies at which our robot vibrates. Achieving behavioral diversity by exploiting mechanical complexity in this manner is a valuable example of morphological computation, in which increasing dynamical coupling can, paradoxically, reduce the cost of control. Given the pervasiveness of both tensegrity and dynamical coupling in biological systems, our hope is that this can lead to a deeper understanding of how mechanically complex living systems at all scales of life move and interact with the world.

Close up the tensegrity robot's body. It is actuated by simple DC vibrating motors attached at the midpoint of three of its six struts. (Photo by Steven Stangle)

Rock and roll!

Links

Tensegrity Wiki on robotics: http://tensegrity.wikispaces.com/Robotics

John Wenz, Popular Mechanics, writing about the robots:
http://www.popularmechanics.com/science/space/nasa/cosmic-concepts-space-balls-rolling-across-titan-15943418

Exploiting Dynamical Complexity in a Physical Tensegrity Robot
to Achieve Locomotion,  By Mark Khazanov, Ben Humphreys, Willam Keat and John Rieffel, DOI: http://dx.doi.org/10.7551/978-0-262-31709-2-ch144