The human hand is an amazing machine that can pick up, move and place objects easily, but for a robot, this "gripping" is a vexing challenge. Opting for simple elegance, a team of researchers from Cornell University, the University of Chicago and iRobot have created a versatile gripper that uses the jamming of particulate material inside an elastic bag to hold on to objects, bypassing traditional designs based around the human hand and fingers.
They call it a universal gripper, as it conforms to arbitrary objects it's grabbing, rather than being designed specifically for particular objects, says Cornell researcher Hod Lipson, associate professor of mechanical engineering and computer science. The research is a collaboration between the groups of Lipson, Heinrich Jaeger at the University of Chicago and Chris Jones at iRobot Corporation. It will be published in the advanced online edition of the Proceedings of the National Academy of Sciences the week of Oct. 25. It also will be the cover article of the journal's Nov. 2 print edition.
In essence, the gripper uses the same phenomenon that makes a vacuum-packed bag of ground coffee so firm; in fact, ground coffee worked very well in the device. But the researchers found a new use for this everyday phenomenon: They placed the elastic bag against a surface and then removed the air from the bag, solidifying the ground coffee inside and forming a tight grip.
Lipson noted that the universality of the gripper makes future applications seemingly limitless - including use by the military to dismantle explosive devices or to move potentially dangerous objects, industrial applications of robotic arms in factories, using the gripper on the feet of a robot that could walk on walls, or making multi-purpose prosthetic limbs.
Here's how it works: Particulate materials are large aggregates of individually solid particles. A special feature of this class of materials, which includes many familiar commodities like sand, grain or ground coffee, is that they can undergo a so-called jamming transition, so they behave more like a solid than a fluid. This happens when particles lose their ability to move past each other.
Crossing the transition
Evacuating the air from a bag of espresso grounds, for example, will move the particles closer until the jamming transition is crossed, at which point the material becomes rigid; releasing the vacuum by unsealing the package unjams the grounds and they can flow.
In the gripper, a latex balloon filled with particulate material is attached to a robotic arm. The balloon presses down in the unjammed state, molds around the desired object and then a vacuum sucks the air out of the balloon, solidifying its grip. When the vacuum is released, the balloon becomes soft again, and the gripper lets go.
"The concept of a jamming transition was developed to provide a unified framework for understanding and predicting behavior in a wide range of disordered, amorphous materials that all can be driven into a 'glassy' state, where they respond like a solid yet structurally resemble a liquid. This includes many liquids, colloids, emulsions or foams, and also particulate matter consisting of macroscopic grains," Jaeger said. "What is particularly neat with the gripper is that here we have a case where a new concept in basic science provided a fresh perspective in a very different area, robotics, and then opened the door to applications none of us had originally thought about."
The team split the work on the project. Eric Brown, a postdoctoral researcher, and Nick Rodenberg, a physics undergraduate, worked with Jaeger on characterizing the basic mechanisms that enable the gripping action. Prototypes of the gripper were built and tested by Lipson and graduate student John Amend as well as at iRobot.
As for the right particulate material, anything that can jam will do in principle, and in early prototypes rice, couscous and even ground-up tires were tried. For the final prototypes, the team settled on coffee because it's light but also jams well. However, there is much room for optimization. What sets the jamming-based gripper apart is that it has performed well with almost any object, including a raw egg or a coin - both notoriously difficult for traditional robotic grippers.
The project was supported by the Defense Advanced Research Projects Agency.