I’d watched with interest some clips of using a vacuum to “solidify” dry coffee grounds in a balloon to effect a robotic gripper, but was finally inspired to actually try to make one as an assistant counselor for a Robotics merit badge at Merit Badge University on 2/25/12.
I found some heavy duty balloons at a local hobby shop marketed as waterproof bags for radios for RC planes. (Who’da thunk it?) At maybe 2″ diameter barely inflated, they were a little smaller than ideal, but that was what I could get on short notice.
The first question was how to fill an inflated balloon? (Actually this is the wrong question: If you have to inflate it to get the amount of coffee you want inside, the balloon is probably too small.) I found a T fitting that would fit the neck of the balloon and the neck of a small flask (with the help of some masking tape). I put “enough” coffee in the flask, and stoppered it with the T – balloon upwards. Using the third leg of the T, I inflated the balloon suitably (obviously pressurizing the flask at the same time). Holding my thumb over the leg I’d blown into, I inverted the whole thing and let the coffee pour into the balloon. Worked fine.
Now – how to keep the coffee grounds out of the vacuum source (whatever that might be…)? First try was a filter to set over the grounds near the mouth of the balloon. I found some near perfect very open, fine net in the sewing room scrap pile. (Might be tulle?) After a few failed tries at poking the stuff into the neck of the balloon, I gave up – with greater respect for ship-in-a-bottle builders. Somebody mentioned a cotton ball, so I stuffed one of those into the balloon. That wasn’t a great idea, either. There’s much too much movement in the balloon to keep anything aligned with the bit of tubing going through the neck.
Final approach (how come it’s always the last thing you try?) was to just stuff the cotton ball into the tubing before putting it into the balloon. Worked fine. Final pressure control (positive and negative wrt atmospheric) was a large syringe that just fit the fat tubing I was using.
For my setup (which included an overfilled balloon), best results were when I pumped maybe 1/2 fl oz of air into the balloon to make it nice and soft so I could squish it firmly around the object to be gripped, then pull the plunger back to solidify the coffee. There’s obviously no actual squeezing of the object from the hardened coffee, but with the sort of sticky balloon surface, it would hold a 9V battery quite nicely. Picking up something like a pen where the balloon could actually get a little bit underneath the object produced better holding. Pushing the plunger back in to let the object drop on the table usually got applause from the scouts watching. Sorry for no pic of that – the balloons were all broken before I got a shot.
All in all a fun exercise. Some thoughts: Get a good sized balloon (obviously not mylar!). You have to push the balloon down to very significantly surround the object to be picked up. The 2 oz syringe I used worked fine for my small balloon. Of course use heavy wall tubing so it doesn’t collapse. Since you have to manually surround the object with the balloon, a surgical glove might be an interesting alternative to a spherical balloon.
Update 3/2/12: I got a package of twenty 12″ balloons at a party store and brought all the stuff to W88 Thursday. I didn’t have time to build anything up before hand, so I assembled one on the table at the space. It worked fine. Here’s the balloon being pushed down around the pickup target. Note that it’s not particularly being squeezed against the target – just pushed down to provide lots of surface contact.
Here’s a successful pickup. But even an unsuccessful pickup leaves a very clear impression of the target. Something like a fat pen that the balloon can fit a little around and under is an ideal target. You do have to squeeze a little to get the grabber around underneath.