Crafting Material Interfaces

Research by Ade, Jill, and Sara

In this post we’ll explore the mechanical process of “jamming” in granular materials. We’ll be focused on an exciting new application of this technology—robotic “gripper” instruments—made possible by exploiting this property, and we’re pulling a lot of the data and photos from the paper, “Universal Robotic Gripper Based on the Jamming of Granular Material.” (Brown, Rodenberg, et al.)

I. What is jamming and why does it matter?

Jamming describes the properties of granular, or “particulate,” materials as they transform from a deformable, flowing shape into a rigid, nearly solid one. This change can happen in a number of substances—from molecular glasses to macroscopic granular materials—and artificial jamming may be employed for mechanical purposes, to create a surface that can transform from an “unjammed,” pliable state to a rigid, solid-like “jammed state” with a vacuum. The most exciting application of jamming is for creating a universal robotic gripper, capable of tune-able sensing and locked gripping of objects. The grip can create a functional robotic arm/hand without the (complicated and often unreliable) use of imitative hand/finger structures used in typical grippers.

II. Other models and precedents

There are several types of grippers available for robotics research:

For the most delicate robotic-humanoid gripping, the dominant research model has sought imitations of the human hand: fingers for sensing and gripping. While this technology has become quite refined and advanced (see the DEKA “Luke” arm, pictured below), these grippers require a large number of controllable joints, the need for force sensing to pick up objects without crushing them, and constant computation of stress alloted to each “finger” of the gripper.

The material explanation of jamming has long been known, but research initiatives are only beginning to extend to the robotic gripper explorations. In a great example of a research reversal, exploiting jamming properties alters the question that has been so challenging to robotics designers in their fixation on the human hand.

III. How does it work?

Granular or particulate materials include a whole range of liquids, colloids, emulsions and foams, as well as grain-like materials, including everyday items like couscous, rice, or coffee. Each of these materials, in a state of increased confinement (created by a vacuum) line up and lock together like small gears, creating a glass-like structure conformed around even the most delicate of objects:

Three physical mechanisms make the gripping action work so well, according to Brown et al: “interlocking geometry between the gripper and object surfaces; static friction from normal stresses at contact; and additional suction effect, if the gripper membrane can seal off a portion of the object’s surface.”

Their research showed success in picking up and gripping a range of objects: small light bulbs, M&M’s, LEDs, bottle caps, plastic tubing, foam ear plugs, and more. The magnitude of holding force was influenced by the objects’ shape, however; and the only objects that could not be gripped were those in which “the gripper membrane could not reach sufficiently around the sides—for example, in hemispheres larger than half the size of the gripper, thin disks, lying flat, or very soft objects like a cotton ball.”

High-Low Tech!

The other great benefit of this application is its low-cost accessibility. Above are John Amend and Hod Lipson of Cornell, with their “high-low” universal gripper, made from coffee grounds and an ordinary balloon; this Science Daily profile details their research.

IV. Resources: Tools, materials

Here’s a video on how to make your own simple gripper. You’ll need a shower head, some 1/4″ tubing, a coupler, some cotton balls, a balloon, a pump, some coffee, and a funnel.