When a human hand grasps an object the hand comforms or adapts to the object and distributes the grip force around it. A conventional prehensor grasps an object in 2 or 3 relatively small places. Because of the limited contact with the object, the mechanical hand requires more force and the precision required in the placement of the contact is critical. Several researchers are pursuing hands that adapt to objects by bending the fingers at multiple places. This paper out of the University of Toronto describes one such aproach involving a set of hinged linkages and this paper out of Stanford describes a simpler approach using a Spectra cord that acts like a tendon and runs through the joints of fingers. Both efforts have shown that as the hand conforms to the variety of object shapes encountered, less overall grip strength is needed to grasp the object.
We’ve been working on a mechanism for adaptive grasp that uses a flexible bag filled with small particles. Normally, the particles can slide past each other and the bag can easily change shape, but when the air is removed from the bag, it squeezes the particles together and they behave like a harder substance. Our first prototype of this concept was the finger of a dishwashing glove filled with sushi rice. Here’s our second prototype of the concept, made from a surgical glove filled with tiny glass beads:
This video (2.2M) shows the glove gripping an object. There is no structure to it yet and it has to be manually positioned onto the object, but when vacuum is applied, it hardens and holds the object securely. This is a good demonstration of adaptive grasp because there is virtually no grip force exerted on the object itself. It is able to conform so readily to the object that it grasps easily even if it is placed without care.
There would be a lot of details to work out on this concept such as the source of vacuum (a piston?), how it actually closes and how to make it durable but it is an interesting direction. I envision a hand made from a simple material that uses vacuum to close around an object and then hardens.
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