Introduction: Sensitive Fingertips

By http://www.plusea.at
About: My work combines conductive materials and craft techniques to develop new styles of building electronics that emphasize materiality and process. I create working prototypes to demonstrate the kinds of electron…

A glove with all-fabric pressure sensors on each fingertip. The idea came from somebody else who wanted to use this as a piano teacher with children to visualize the difference between "p" piano (soft) and "f" forte (hard).

The pressure sensitive layers of fabric in the fingertips of these gloves are stretchy so that they can fit tightly. The sensors are made from stretch conductive fabric and piezoresistive Eeonyx fabric. The glove is connected to the Arduino via metal snaps and a fabric cable made from sewn conductive thread traces.

HOW THE SENSOR WORKS
The piezoresistive effect describes the changing electrical resistance of a material under mechanical pressure. Eeonyx coats a range of anti-static woven and non-woven fabrics in an inherently conductive polymer, giving them piezoresistive properties. By adhering two traces of stretch conductive fabric parallel to one another across the fingertip and then attaching a piece of piezoresistive stretch fabric on top of these, one is able to measure the change is resistance between the two conductive traces when pressure is applied through the piezoresistive material.

This is the first working prototype, things to be improved include:

  • Integrating the sensors better around the shape of the fingertip, possibly knitting the glove and including conductive and resistive yarns to do so in a three layer knit
  • Making sure the sensor reacts well not only to holding down pressure, but to the tapping of piano keys
  • Making sure all this works on the scale of a child's hand
  • Improve the software and visualization side

More information about the techniques, materials and tools used in this Instructable can be found in our KOBAKANT database How to get what you want >>
http://www.kobakant.at/DIY

Videos

Step 1: Materials and Tools

MATERIALS:
  • Eeonyx SL-PA coated piezoresistive stretch fabric (RL-5-129) from http://www.eeonyx.com
  • Stretch Conductive Fabric from www.lessemf.com/fabric.html
  • 117/17 2 ply conductive thread from www.lessemf.com/fabric.html
or www.sparkfun.com
TOOLS:
  • Sewing needle
  • Sewing machine
  • Scissors
  • Pen and paper
  • Iron
  • Cutter knife
  • Pliers
  • Soldering iron

Step 2: Stencil Tracing and Cutting

Download the stencil from here >>
http://kobakant.at/downloads/stencils/SensitiveFingertips.pdf
And trace it on to stretchy jersey fabric. You might need to scale the stencil up or down to fit your hand as this stencil was designed for my own hand, which is about 20cm in circumference and 18cm from the tip of my middle finger to the base of my hand. Wow my hand is size XL!
>> http://www.glove.org/glovemeasure.htm

Once you have traced the stencil, cut the pieces out.

1 x palm
1 x thumb
1 x pointer
1 x index finger
1 x ring finger
1 x little finger

Step 3: Conductive Traces

Take a piece of the stretch conductive fabric turn on your iron, making sure it is not too hot to burn the conductive fabric (golden discoloration is okay). Fuse a piece of fusible interfacing to the smoother side of the piece of stretch conductive fabric. Cut the conductive fabric into 5mm wide traces that are long enough to reach from your fingertips to the back of your hand - spiraling around your fingertip. About 20cm long.

Lay out the finger and thumb pieces and fuse the conductive fabric strips as shown in the photo. leaving the ends long, so that they can be further connected in step 5!

Step 4: Sewing

Thread your needle with regular thread and sew the pieces together - take a look at the following illustrations on how they should be sewn together.
Rather than sew the edges together in a running stitch, I used a hemming stitch, but did not fold the fabric over. This does not give the most beautiful stitches, but it does reduce the amount of fabric.
>> http://www.sewdresses.com/wp-content/uploads/2f7cf27e287fe92.gif

Depending on which way around the fingertip parts are facing (they must all face the same way!) will determine weather this glove is for your left or right hand.

Step 5: Conductive Trace Again

On the back of the hand of the glove mark the position of the connection poppers. For this use the plug stencil that can be downloaded here:
>> http://kobakant.at/downloads/stencils/SensitiveFingertips-plug.pdf

You can also download the original size of the schematic illustration from here:
>> http://farm4.static.flickr.com/3401/3659523353_6ae26c39fb_o_d.jpg

Lay out the glove and continue to fuse the fabric traces from the fingers making your way towards the back of the hand to the appropriate marking. To know which finger trace should go to which marking, see the schematic illustration. One of the fabric traces from each sensor will all connect together and to the Ground (GND, -).

T = thumb
P = pointer
I = index finger
R = ring finger
L = little finger
T, P, I, R, L --> GND

Once you have fused all the traces into place, finish off by punching male poppers through the ends of the conductive fabric traces on the back of the hand, where the plug will attach.

Step 6: Isolating

Cut strips from the jersey fabric with which you want to isolate your conductive traces. You don't have to use a fabric on top of the glue if you don't want to, but the glue tends to remain sticky and is hard to apply super evenly and so tends to look a bit messy. You can apply flour or baby powder to the glue once it has dried to get rid of some of the stickiness. Covering the glue with fabric strips is the easy way out. And it doesn't look bad.

Get out the stretchy fabric glue and apply it about 1mm thick and evenly to the conductive traces. Makes sense to do one side at a time. Wait for the glue to dry for about 10-15 minutes (when the bluish sheen is gone) before adding the fabric strips, otherwise the glue will seep right through the fabric.

Make sure you leave the beginnings of the conductive traces on the fingertips exposed! Otherwise you can't make the sensor.

Step 7: Eeonyx Sensor Fabric

Eeonyx sensor fabric
To make the sensors you'll need to cut ovals of the Eeonyx RL-5-129 SL-PA coated piezoresistive stretch fabric. The ovals should be big enough to cover the fingertips, the same size as the tip of the finger stencil.

Step 8: Pullup and Low-pass Filter Circuit

Cut a piece of perfboard 8 x 9 holes big. Copper lines running the shorter distance. Solder one 4.7uF capacitor (low-pass filter) and one 50K ohm resistor (pull-up resistor) between each input and the +.

Please take a close look at the photos and illustrated diagram to see how things are connected.

Step 9: Plug and Cable

Trace the cable stencil twice onto the piece of non-stretch fabric. Cut out small squares of stretch conductive fabric and fuse them to the positions of the Xs marked on the cable stencil.

Prepare your sewing machine with conductive thread. Wind the 117/17 2ply conductive thread onto a bobbin. For the top thread of the sewing machine you can use any colour you like. Preferably make the GND cable stand out by choosing a different colour than the rest.

Sew from the patches of conductive fabric all along the meter of non-stretch fabric to the other end (see photos and stencil), leaving about 10-15 cm extra of the conductive thread that later need to be sewn to the perfboard.

Make sure that the threads do not fray or make any sort of contact between one another. Their spacing should be about 2mm so that they line up with the spacing of the holes on the perfboard.

When all the threads have been sewn, pierce female poppers through the patches of conductive fabric. So that the face of the popper is NOT on the side of the conductive thread.
If the fabric you used is rather thin, you can reinforce it in the head part with an extra layer and some fusible interfacing. This will stop the poppers from tearing the fabric when you connect and remove the plug.

You can also add some fusible interfacing on top of the conductive side of the threads, so that when you have sewn and turned the cable inside out you can fuse this fusible by ironing over it and it will stop the threads from making bad contacts within the cable.

Lay right sides together (conductive stitches facing outwards) and sew the pieces for the plug and cable together, leaving the end with the loose conductive threads open. Turn the cable inside out. And close by hand.

Stitch the loose conductive threads to the perfboard. Make sure none of them touch each other. You can use some of the fabric glue to isolate them when all of them have been sewn.

Step 10: Connecting and Running the Application

From the tips of the fingers, to the poppers on the glove, to the poppers on the plug to the threads on the cable to the perfboard, to the headers and into the Arduino the following should match up:

Analog Input 0 = Ring finger
Analog Input 1 = Index finger
Analog Input 2 = Little finger
Analog Input 3 = Pointer finger
Analog Input 4 = Thumb

For Arduino microcontroller code and Processing visualization code please look here >> http://www.kobakant.at/DIY/?cat=347

Plug everything together and run the application.
See where the range of the sensors for the individual fingers lie and you can set your thresholds in the code. Run the application again to work with new thresholds. Press 'g' (graph) and 'd' (draw) to toggle between raw input and thresholded input views.

I hope everything works and please let me know if anything is unclear.
Enjoy!

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