Robotic Foam Hand

This is how to make a home brew robotic hand using foam.

This project was made for Humanoids 16-264, with thanks to Professor Chris Atkeson and TA Jonathan King

A lot of the supplies for the physical rig that held everything in place were items found around the house, I used cardboard, tape, wooden rods, thread, and an old bedsheet. Almost all of these could be replaced with other items.

These are the more specific supplies for the project:

Arduino (generic brand) $14

https://www.amazon.com/ELEGOO-Board-ATmega328P-ATM...

bread board, wires, etc. kit $16

https://www.amazon.com/REXQualis-Electronics-tie-...

servos (pack of 2) x3 $10 each

https://www.amazon.com/KAILEDI-Arduino-Motors-Wal...

flex foam-it X $27.78 for 2 pound trial size (can find distributor through link)

https://www.smooth-on.com/products/flexfoam-it-x/

plasticine clay $12.94

https://www.amazon.com/Sargent-Art-Plastilina-Mod...

Use the plasticine clay to cover your hand for the mold. The denser the layer of clay the better the hand will turn out. You will then have to carefully wiggle your hand out of the mold.

If this were to be done with a more complicated item, something you cant wiggle out, the mold may have to be separated into multiple pieces. Since plasticine clay doesn't dry, I was able to reuse it multiple times for separate attempts, however this trait also prevents the clay from shaping something more complicated that a 3d printer would be able to achieve. Because the clay is soft, it slowly moves out of place, and trying to separate it into multiple parts becomes a messy procedure. For our purposes though, it fits the bill. Additionally, the clay can also pick up small details such as finger nails or folds in the skin.

Use Flex Foam-it X to fill the mold of your hand. Make sure to wear gloves for this and ideally find an open environment. Flex Foam-it X expands up to 6 times the amount of product you use, so be careful when using it, and allow space for overflowing. If you use less than intended, you can run a second round that will bind to the first.

To use Flex Foam-it X, mix parts A and B in a 1:1 ratio, again using roughly 1/6th the volume you intend to fill. Carefully pour the solution into the mold, making sure to fill each finger. The mix starts to heat up once you stir it and starts expanding within 30 seconds, so pour it in before that happens.

After 2 hours, the foam should be set, and you will be able to remove the mold. The foam hand is sturdy, but its best to be careful when removing the clay to avoid losing a finger.

Before I used clay I tried filling up a plastic glove as a test, but it ended up a little wonky looking. Also the plastic "skin" broke when I tried attaching a thread tendon. I used the plasticine 3 times to create hands, but found the second attempt to be the most successful. Even though it seems that the plasticine can be reused endlessly, the parts of the clay that were in contact with the foam took on a pale yellow color.

Using everyday thread, cloth, needles and scissors, make a glove for the foam hand. This step is crucial because we need to create a way for the fingers to bend. Just like how pulling a loose thread can cause a shirt to bunch up, we can pull a thread to get our fingers to bend.

The cloth looks like skin, but its purpose here is to keep our string "tendons" in place. Use a button to fasten a thread to the tip of a finger, then sew in the thread "tendon" in the spots of the finger you want it to bend in. You might want to experiment with the spacing to figure out what looks best.

Put everything on a board so that it all stays in place. This can be done in any number of ways, but for our purposes, cardboard works fine.

I got continuous rotation servos, and intended to make wheels that would spool the thread to bend the fingers. For some reason however, the servos acted like the normal kind, and instead of taking speed took degrees, so they would only moved between 0 and 180. Because this wasn't enough to get the fingers to bend all the way, I attached a wood stick to the servo to increase the length it would pull the tendon. It created a bit of a spacing issue but worked out.

I attached the hand by inserting wooden sticks into the base and taping that to the cardboard.

Nows a good time to begin putting all the mechanical parts together. If you have a basic understanding of circuits, this shouldn't be too hard to follow. I use a lot of buttons to control each fingers bent and unbent state. I included a couple photos of some of the earlier versions of the setup.

If you want to get more involved, it would be cool to play around with hand tracking or motion sensors. You could also add more tendons to the fingers to give it more dynamic motion or use tons of pneumatic muscles to get closer to the anatomy of the human hand.

This code is made with the Arduino software, using the Arduino Uno board, and corresponds to the set up earlier. Most of the angles the servos move between are 0 to 180, but because some of the servos didn't need to travel that far to bend its finger, I manually set them to work with the rig. This is something you should do testing to figure out what angles work best. I included an earlier version of the code when I was testing things out.

There are a bunch of different ways to do this. A much larger project using this would be translating text into sign language by adding another hand, a few more tendons, arms and elbows, a dictionary of images of signs, and a software that can take those images and find the positions and movements of the hands.

Once everything is put together, you should have your own home brew hand!