The usual strategy when designing a robot hand is to try to replicate the human hand. However, in 2010, researchers at Cornell University and the University of Chicago developed a unique approach: They created an amorphous gripper that can mold itself to the object to be picked up. It’s very versatile and it’s easy to build.
This gripper works because of a process called “jamming”. When a granular material — such as coffee — is compressed, it becomes very rigid. As the pressure increases, so does the amount of friction between the individual grains. This effectively locks the grains in place.
You may have observed this phenomenon while handling bags of ground coffee. A vacuum-packed bag of coffee is rock-hard as long as the seal remains intact. But as soon as the seal is broken, the coffee becomes soft and pliable and can be poured like a fluid. This process happens with many granular materials, such as rice, couscous, and even sand.
We’re using this process to make an amorphous robot gripper. A balloon filled with coffee is attached to an air hose; when balloon is slightly pressurized the grounds are loose and easily rearranged. By pressing the balloon against an object, the grounds will move around it and take its shape. But when the air is sucked out of the balloon, the grounds are compressed and grip the object. The rubber surface of the balloon also helps to keep a hold on the object.
The first thing you need to do is fill the balloon with coffee. To do this, attach the balloon to the end of a short tube or pipe, and insert the funnel into the other end.
Scoop about a tablespoon of dry, ground coffee into the funnel and it will pour down into the balloon. Then remove the funnel and blow into the tube to partially inflate the balloon. This will allow all the coffee to fall to the bottom of the balloon. As you slowly let the air back out, the coffee will remain trapped in the balloon. Then insert the funnel back into the tube and repeat the process.
Continue adding coffee. Periodically, set the balloon inside the funnel to check its size. You want the balloon to stick out about one inch past the edge of the funnel. Once you have enough coffee in the balloon, you can remove the balloon from the tube.
Now we need to put the balloon in the funnel and insert the neck of the balloon through the narrow opening. Unfortunately most kitchen funnels have a narrow section that is several inches long. This makes the process very difficult. So to make it easier, I cut the narrow section of the funnel so that it is only 1/2 inch long. You can do this with any sharp knife. After cutting the funnel, try to smooth off any rough edges.
Insert the neck of the balloon through the funnel and wrap it around the opening on the other side. To hold it in place, I applied small strips of duct tape.
We want the ground coffee to stay inside the balloon. To prevent it from falling out, I attached a small piece of fabric to the opening, to act as a rough filter. You want to use a fabric that breathes easily so that the air pump will be able to quickly move air in and out of the balloon.
Tightly wrap the fabric around the opening of the funnel, then secure it in place with small strips of duct tape.
Now you need to attach the air hose to the funnel. The easiest way to do this is to hold the funnel up in the air hose and attach them together with several layers of duct tape. You want to make a (mostly) air tight seal, so feel free to use as much tape as you want.
Now you are ready to use your universal gripper to pick up objects. Start by partially inflating the balloon. This will make the grains loose so that they will freely move around the object. Then gently press the balloon on top of the object. Now suck the air out of the balloon. Continue pressing down on the object as the balloon deflates. The balloon will shrink and the grounds will lock in place around the object.
When all the air is sucked out of the balloon, you should be able to pick up the object. As long as the vacuum is maintained, the universal gripper should continue to hold the object firmly. To release the object, simply break the seal and let some air back into the balloon; the object will then fall from the gripper. If you quickly blow air back into the balloon the gripper will forcefully eject the object. You can use this to shoot small objects across the room.
This kind of gripper is ideal for pneumatically powered robots; all you have to do is hook the gripper up to the robot’s air line and you will be able to manipulate a wide variety of objects with ease.
