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October 26th, 2010

Video(s) of the week: Granular material techniques reboot robots’ grip

Published on October 26th, 2010 | By: pwray@ceramics.org

A granular “blob” instead pinchers or fingers at the end of robot arms? Researchers from the University of Chicago, Cornell and iRobot think they have developed a markedly simple step forward in creating a universal gripper for robotic applications that exploits how granular material can clump or “jam” around random shapes when subjected to a vacuum.

In a new paper in the Proceedings of the National Academy of Sciences, the group reports that their system can grasp and hold objects without a sensory feedback system, opening up tremendous opportunities “for the design of simple, yet highly adaptive systems that excel at fast gripping of complex objects.”

In brief, the approach entails taking a sack containing a mass of granular material (see blue blob, above) and pressing onto the desired object. The sack is subjected to a vacuum that causes the granular mass to transition from a flowable state to a hardened shape. Friction, suction and interlocking mechanisms combine to do the job. The authors say, “We find that volume changes of less than 0.5% suffice to grip objects reliably and hold them with forces exceeding many times their weight.”

Here is how they describe their system:

“The unique properties of a jamming gripper derive from the fact that loose grains in a bag sit at the threshold between flowing and rigid states. This enables the gripper to deform around the target in the unjammed, malleable configuration, then harden when jamming is initiated. In the vicinity of the jamming transition very small modifications of the packing density can drive dramatic changes in the mechanical response. Thus, increasing the particle confinement slightly, e.g., by applying a vacuum, enables the gripper to gain remarkable rigidity while almost completely retaining its shape around the target.”

If you find this less than intuitive, think about sitting in a beanbag chair. The foam beads inside flow around your shape but eventually settle in to “grip” your body. Now imagine a vacuum being applied to the chair. It would, in fact, harden, retain its shape, and might even provide some resistance if you tried to stand. In fact, I experienced this a view years back when a radiologist used a beads-and-vacuum system to make a tight mold of part of my body in preparation for some radiation treatments.

As shown in video above, gripping often requires only coming in contact with a small part of the objects surface, but, ultimately, the magnitude of the holding force affected by the objects’ shape. They acknowledge that there were a few instances where their gripping system failed, such as when the gripper membrane could not reach far enough around the sides of an object (such as hemispheres larger than about half the size of the gripper) or for thin disks lying flat, or for very soft objects (such as cotton balls).

Lead author, Eric Brown, as part of UC’s Jaeger Group, has been investigating the strength of jammed grains for some time and explains that besides stationary applications, he and others have been working with iRobot to develop “Chembot,” an odd moving, morphing mobile robot. See a demonstration below:


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